Olfactory receptor neurons (ORNs) require rapid signal termination mechanisms in order to faithfully report odors that often occur at low and rapidly fluctuating levels. These mechanisms must overcome the obstacle of extremely slow diffusion of odors out of olfactory tissues compared to the rate of environmental fluctuations. In species from insects to mammals, primary olfactory neurons are embedded in non-neuronal auxiliary cells. These cells are thought to mediate odorant clearance, which may contribute to signal termination, but there are few demonstrations of their in vivo effects on neuronal activity. In insects, small stereotyped groups of ORNs and auxiliary cells are encapsulated in hair-like structures known as sensilla. My long term objective is to exploit this organization in the genetic model Drosophila to determine if and how auxiliary cells affect odor encoding and behavior. I recently identified an auxiliary cell transporter that affects responses to ammonia, an odor that attracts many insect vectors of disease. Unexpectedly, loss of this transporter Amt did not simply impair response termination, but instead entirely abolished responses in a particular class of olfactory sensilla. In this R03 proposal, I aim to decipher the relationship between odorant uptake and olfactory receptor sensitivity using ammonia detection as a model system. I will take a multidisciplinary approach combining electrophysiology, molecular biology, genetics, and microscopy. In Aim 1, I will test my hypothesis that ammonia uptake serves to prevent olfactory receptor desensitization. Experiments in Aim 2 are designed to investigate the relationship between olfactory receptors' sensitivity to ammonia and their susceptibility to desensitization. In Aim 3, I will determine whether ammonia responses in other sensilla are supported by Rh50, the only other ammonia transporter in insects, or whether their ammonia responses are independent of ammonia uptake. Based on my preliminary data, I expect that this research will point to odorant uptake as an essential regulator of ORN sensitivity and more firmly establish the integral role of auxiliary cells in olfactory circuit activity. These results will serve as a foundation for an R01 proposal examining how such clearance mechanisms are generalized for the enormous number of odors that olfactory systems can detect.